Process for the preparation of difluoromethane

ABSTRACT

Difluoromethane is prepared by reacting dichloromethane and hydrogen fluoride in a liquid phase in the presence of a fluorination catalyst under reaction conditions in which the reaction pressure is between 1 and 10 kg/cm 2  ab., and the reaction temperature is between 50 and 150° C., provided that the selected reaction temperature is higher than a temperature at which hydrogen fluoride is not liquefied under the selected reaction pressure. 
     Under the above conditions, the conversions of dichloromethane and HF are very high, and amounts of by-products other than R30 are very low, typically less than 0.1% per difluoromethane, when the unreacted materials are recycled. 
     A material of a reactor is hardly corroded by the reaction using an antimony chlorofluoride and HF which are highly corrosive, as long as the above conditions are maintained.

FIELD OF THE INVENTION

This application is a 35 USC 371 National Stage Filing ofPCT/JP95/01122, published as WO/95/35271 on Dec. 28, 1995.

The present invention relates to a process for the preparation ofdifluoromethane comprising fluorinating dichloromethane with hydrogenfluoride in a liquid phase in the presence of a catalyst.

PRIOR ART

Difluoromethane (hereinafter referred to as "R32") attracts attention asa substitute refrigerant for chlorodifluoromethane which is used as arefrigerant for air conditioners.

It is known that R32 is prepared by reacting dichloromethane(hereinafter referred to as "R30") with hydrogen fluoride (HF) in a gasor liquid phase in the presence of a catalyst.

U.S. Pat. Nos. 2,749,374 and 2,749,375 disclose a process for thepreparation of R32 comprising reacting R30 and HF in a liquid phase at atemperature between 110 and 175° C. in the presence of an antimonychlorofluoride catalyst (SbCl_(x) F_(y) in which x +y=3,y/(x+y)=0.8 andSb(V)>5%). However, this process produces a large amount of by-productsof R40 series such as monochlorometane (hereinafter referred to as"R40") and fluoromethane (hereinafter referred to as "R41") which areundesirable impurities in addition to R30 and decrease the yield of R32.It is very important for the reaction system not to corrode thematerials of reaction apparatuses in the production of R32, but HF andhalogenated antimony are known to corrode such the materials. Neither ofthe above U.S. Pat. Nos. describe that the materials of reactionapparatuses had the corrosion resistance when the reaction was performedunder the above conditions.

U.S. Pat. No. 4,138,355 discloses the addition of an equimolar amount ofantimony trihalide to antimony pentahalide to prevent the corrosion of areactor caused by a mixture of halogen-containing organic compounds withHF and antimony pentahalide. However, the composition of the catalystmay vary with the progress of the reaction since the amount of antimonytrihalide increases due to the degradation of the catalyst.

JP-A-59-231030 discloses a process for the preparation of R32 comprisingreacting R30 and HF in a gas phase at a temperature of 200° C. in thepresence of aluminum fluoride or chromium fluoride as a catalyst. Thisprocess is not economically advantageous since the reaction temperatureis as high as 200° C. and the gas phase reaction requires morecomplicated apparatuses than the liquid phase reaction.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above problemsassociated with the prior art and to provide a process for preparing R32economically and safely.

Accordingly, the present invention provides a process for thepreparation of R32 comprising reacting R30 and HF in a liquid phase inthe presence of a fluorination catalyst, wherein the reaction pressureis between 1 and 10 kg/cm² ab., and the reaction temperature is between50° and 150° C., provided that the selected reaction temperature ishigher than a temperature at which HF is not liquefied under theselected reaction pressure.

DETAILED DESCRIPTION OF THE INVENTION

Examples of the fluorination catalyst used in the process of presentinvention are antimony chlorofluorides, titanium chlorofluorides, tinchlorofluorides, and the like. A preferable catalyst is an antimonychlorofluoride of the general formula:

    SbCl.sub.x F.sub.y

wherein x+y=5. In particular, the antimony chlorofluoride of the aboveformula in which y is a number of between 1 and 4 is preferable. When yis less than 1 (one), the conversion of R30 is low and the yield of R32per unit weight of the catalyst decreases. Therefore, a large amount ofthe catalyst should be used. Nevertheless, the antimony catalyst of thegeneral formula in which the index y is between 0.5 and 1 may be used.When the index y is larger than 4, the process is less economical sincea large amount of HF should be recycled. The index y is preferablybetween 2 and 3.

The antimony chlorofluoride is a chlorofluoride of pentavalent antimonyprepared in situ by partial fluorination of antimony pentachloride. Asdescribed above, the index y in the antimony chlorofluoride of the aboveformula is preferably between 1 and 4. The ratio of x to y may vary andtherefore the catalyst may lose its activity as the reaction proceeds inthe conventional processes. However, the index y can be maintained inthe above range under the reaction conditions according to the presentinvention.

The fluorination catalyst is used in an amount of between 10 and 90 mole% in the liquid reaction mixture. When the amount of the catalyst isless than 10 mole %, the residence time of the reaction gas isprolonged, the amount of the R40 series impurities increases, andtherefore the product must be purified in some cases. When the amount ofthe catalyst exceeds 90 mole %, the amount of the organic materials istoo small, the amount of the catalyst entrained in the splashed liquidincreases, and therefore pipes and other equipments tend to be clogged.The more preferable amount is between 40 and 70 mole %, while it maydepend on the reaction temperature.

The reaction pressure is between 1 and 10 kg/cm² ab., preferably between5 and 10 kg/cm² ab.

The reaction temperature should be in the range between 50° and 150° C.,provided that the selected reaction temperature is a temperature atwhich HF is not liquefied under the selected reaction pressure.Preferably, the reaction temperature is at least 5° C. higher than theboiling point of HF under the selected reaction pressure. The reason whythe present invention sets this limitation is that the corrosionresistance of the reaction apparatuses severely deteriorates andtherefore the process cannot be safely performed if liquid state HF ispresent in the liquid reaction mixture. R30 is present mainly in theliquid phase, while HF is present mainly in the gas phase in the processof the present invention.

The process of the present invention in a preferred embodiment can beperformed by the following steps:

(1) First, the fluorination catalyst is charged into a reactor.

(2) Then, R30 and HF are charged into the reactor to effect thereaction. The reaction is performed under the above described reactionconditions, and thus R32 and chlorofluoromethane (hereinafter referredto as "R31") which is an intermediate are produced.

This reaction can be performed in a generally known apparatus. Ingeneral, the reactor is required to be supplied with the raw materials(R30 and HF) and recycled materials (R31, R30 and HF) which will beexplained below, in the liquid or gas state and to heat or cool theliquid reaction mixture sufficiently. Furthermore, the reactor isrequired to facilitate the contact between the reactants by a suitablemixing method. In addition, the reactor should maintain the reactiontemperature in the range in which HF in the liquid reaction mixture isnot liquefied under the selected pressure, even when HF is charged inthe liquid state into the reactor.

(3) A portion or whole of the reaction mixture is recovered from thereactor. To this end, a reflux column and a reflux condenser areattached to the reactor, and the reaction mixture is recovered in theform of a refluxed condensed liquid or uncondensed gas. The provision ofthe reflux column and reflux condenser will prevent the splash of thecatalyst together with the reaction mixture.

(4) The recovered reaction mixture is separated to obtain a mixturecontaining mainly R32 as the reaction product and hydrogen chloride andanother mixture containing unreacted R30, HF and intermediate R31. Thisseparation can be effected by distillation since R32 and hydrogenchloride have relatively low boiling points while R30, HF and R31 haverelatively high boiling points.

(5) R32 is isolated from the mixture containing R32 and hydrogenchloride by a conventional method such as distillation or washing withwater.

(6)The mixture containing unreacted R30, HF and R31 is reused byrecycling it to the reactor.

The above process is preferably performed continuously, while it may beperformed by a batch process.

Preferable materials used for constructing the reactor which is used inthe process of the present invention are Hastelloy C-22, NAR-25-50MTi,double phase stainless steel, SUS, carbon steel and the like. Amongthem, Hastelloy C-22 and NAR-25-50MTi are particularly preferable.

The present invention will be illustrated by following Examples andComparative Examples.

EXAMPLE 1

R30 was continuously fluorinated at 100°C. under 6 kg/cm² G in a 600 mlreactor made of Hastelloy C-22 to which a reflux column and a refluxcondenser had been attached.

That is, R30 and HF were continuously supplied into the reactor whilethe reaction product was continuously recovered through the refluxcondenser. SbCl₂ F₃ was used as the catalyst, and the concentration ofthe catalyst in the reaction liquid was kept constant so that thiscatalyst composition was maintained.

Sample pieces of various metals for corrosion test, which had beendefattedwith acetone and weights and sizes of which had been measured,were dipped in the reaction liquid during the continuous fluorination.The corrosion rate was calculated from the weight of each sample piecemeasured after 8 hours and the surface loss calculation. The results areshown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Metal              Corrosion rate (mm/year)                                   ______________________________________                                        Carbon steel       0.75                                                       SUS 316            0.25                                                       Double phase stainless steel (DP-3)                                                              0.07                                                       NAR-25-50MTi       <0.01                                                      Hastelloy C-22     <0.01                                                      ______________________________________                                    

It is understood from Table 1 that the metal materials which are usedfor the construction of the reactor are not excessively corroded underthe reaction conditions in the process of the present invention.

EXAMPLE 2

The fluorination was carried out in the same manner as in Example 1except that the reaction pressure was changed. In this example, thepressure of 15 kg/cm² G was the pressure condition under which HF wasliquefied in the reaction mixture at the reaction temperature of 100°C., while 4 kg/cm² G was the pressure condition under which HF was notliquefied. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Metal        4 kg/cm.sup.2 G                                                                            15 kg/cm.sup.2 G                                    ______________________________________                                        Carbon steel    0.7 mm/year                                                                             Unmeasurable due to                                                           heavy corrosion                                     SUS 316        0.21 mm/year                                                                             Unmeasurable due to                                                           heavy corrosion                                     Double phase   0.03 mm/year                                                                             16.6 mm/year                                        stainless steel                                                               (DP-3)                                                                        NAR-25-50MTi <0.01 mm/year                                                                              19.5 mm/year                                        Hastelloy C-22                                                                             <0.01 mm/year                                                                              10.3 mm/year                                        ______________________________________                                    

It is understood from Table 2 that the corrosion of the metals used fortheconstruction of the reactor is suppressed under the pressurecondition under which HF is not liquefied according to the presentinvention, while the metals are excessively corroded under the pressurecondition under which HF is liquefied, when the reaction temperature isthe same.

EXAMPLE 3

The fluorination was carried out in the same manner as in Example 1except that the reaction temperature was changed. In this example, 80°C. was the temperature condition at which HF was liquefied in thereaction mixture under the reaction pressure of 6 kg/cm² G, while 120°C. was the temperature condition at which HF was not liquefied. Theresults are shown in Table

                  TABLE 3                                                         ______________________________________                                        Metal        80° C.  120° C.                                    ______________________________________                                        Carbon steel Unmeasurable due to                                                                            0.84 mm/year                                                 heavy corrosion                                                  SUS 316      Unmeasurable due to                                                                            0.33 mm/year                                                 heavy corrosion                                                  Double phase 16.2 mm/year      0.1 mm/year                                    stainless steel                                                               (DP-3)                                                                        NAR-25-50MTi 18.5 mm/year   <0.01 mm/year                                     Hastelloy C-22                                                                              9.7 mm/year   <0.01 mm/year                                     ______________________________________                                    

It is understood from Table 3 that the corrosion of the metals used fortheconstruction of the reactor is suppressed under the temperaturecondition at which HF is not liquefied according to the presentinvention, while thematerials are excessively corroded under thetemperature condition at whichHF is liquefied, when the reactionpressure is the same.

EXAMPLE 4

The fluorination was carried out in the same manner as in Example 1except that the index y in SbCl_(x) F_(y) (x+y=5) was changed. Theresults are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Metal            y = 1       y = 4                                            ______________________________________                                        Carbon steel       0.69 mm/year                                                                              0.78 mm/year                                   SUS 316            0.21 mm/year                                                                              0.29 mm/year                                   Double phase stainless steel                                                                     0.05 mm/year                                                                              0.09 mm/year                                   (DP-3)                                                                        NAR-25-50MTi     <0.01 mm/year                                                                             <0.01 mm/year                                    Hastelloy C-22   <0.01 mm/year                                                                             <0.01 mm/year                                    ______________________________________                                    

It is understood from the above results that the metals used for theconstruction of the reactors are not excessively corroded under thepressure and temperature conditions under which HF is not liquefied, ifthe index y of SbCl_(x) F_(y) changes.

EXAMPLE 5

The reaction in this Example was performed using the same apparatus asusedin Example 1 but an equipment for recycling unreacted materials(R31, R30 and HF) was attached to the apparatus.

The specific amount of the catalyst of SbCl_(x) F_(y) (x+y=5) in which yhad been adjusted to 2 was charged in the apparatus, and the molarratioof supplied HF to supplied R30 was adjusted to about 2. Thereaction pressure was 6 kg/cm² G.

The reaction temperature was kept at 90°0 C. which is 5° C. higher thanthe boiling point of HF under the pressure of 6 kg/cm² G,that is, 85°C.,so that HF was not liquefied under the pressure of 6kg/cm² G.

The concentration of SbCl_(x) F_(y) in the reaction mixture was adjustedto 50 mole %.

The reaction mixture was recovered through the reflux condenser, andseparated to obtain the reaction product (the mixture of R32 and HCI)and the unreacted materials (the mixture of R31, R30 and HF) as follows:

The recovered reaction mixture was introduced in a distillation columnmadeof SUS 316 and distilled under 5 kg/cm² G.

The mixture consisting mainly of R32 as the reaction product and HCI wasallowed to flow out from the condenser while maintaining the toptemperature at about -26° C., and the mixture of unreacted R31, R30andHF was discharged from the bottom of the distillation column. Themixture of the unreacted materials was recycled to the reactor.

When the reaction was stabilized, the organic materials and acidscontainedin the reaction mixture, the exit gas from the reflux condenserattached tothe reactor, the exit gas from the recycling equipment andthe recycled liquid were analyzed, and their compositions weredetermined. The results are shown in Table 5.

The composition of the antimony chlorofluoride used as the catalyst wasanalyzed, and the index y was 2.2.

                  TABLE 5                                                         ______________________________________                                                                     Exit gas                                                            Exit gas  from the                                                 Reaction   from the  recycling                                                                            Recycled                                          mixture    condenser equipment                                                                            Liquid                                    Component                                                                             (mole %)   (mole %)  (mole %)                                                                             (mole %)                                  ______________________________________                                        HCl                33.5      67.0                                             HF                 27.7       0.6   54.9                                      R32     0.4        16.7      32.2   1.2                                       R31     4.8        10.0       0.2   19.9                                      R30     46.2       12.0             24.0                                      R40's                67 ppm   131 ppm                                         SbCl.sub.x F.sub.y                                                                    48.6                                                                  ______________________________________                                    

The above results indicate that the concentration of the catalyst in theliquid reaction mixture and the index y of the catalyst were controlledstably, that the conversions of R30 and HF in the exit gas from therecycling equipment were very high and both higher than 99 mole %, andthat the amounts of the by-products other than R30 were very low, thatis,less than 0.1% per produced R32.

EFFECTS OF THE INVENTION

The reactor made of the metal such as Hastelloy C-22 or NAR-25-50MTi isnotcorroded by the reaction using the antimony chlorofluoride and HFwhich arehighly corrosive, when the reaction is carried out under theconditions according to the present invention. The conversions of R30and HF can be increased greatly, and the amounts of the by-productsother than R30 are very low, typically less than 0.1% per produced R32,when the unreacted materials are recycled.

What is claimed is:
 1. A process for preparing difluoromethane comprising reacting dichloromethane and hydrogen fluoride in a liquid phase in the presence of a fluorination catalyst, wherein the reaction pressure is between 1 and 10 kg/cm² ab., and the reaction temperature is between 50° and 150° C., provided that the selected reaction temperature is a temperature at which hydrogen fluoride is not liquefied under the selected reaction pressure.
 2. The process according to claim 1, wherein the fluorination catalyst is an antimony chlorofluoride of the general formula:

    SbCl.sub.x F.sub.y

wherein x+y=5.
 3. The process according to claim 2, wherein y is a number of between 1 and
 4. 4. The process according to claim 2, wherein y is a number of between 0.5 and
 1. 5. The process according to claim 1, wherein the content of the catalyst in the liquid reaction mixture is between 10 and 90 mole %.
 6. The process according to claim 1, wherein the reaction temperature is at least 5° C. higher than the boiling point of hydrogen fluoride under the selected reaction pressure.
 7. The process according to any claim 1, which comprises the steps of:(1) charging the fluorination catalyst into a reactor, (2) charging dichloromethane and hydrogen fluoride into the reactor and reacting them, (3) recovering a portion or whole of the reaction mixture from the reactor, (4) separating the recovered reaction mixture to obtain a mixture containing mainly difluoromethane as the reaction product and hydrogen chloride and another mixture containing unreacted dichloromethane, hydrogen fluoride and intermediate chlorofluoromethane, (5) isolating difluoromethane from the mixture containing difluoromethane and hydrogen chloride, and (6) recycling the mixture containing dichloromethane, hydrogen fluoride and chlorofluoromethane.
 8. The process according to claim 7, which is performed continuously.
 9. The process according to claim 1, wherein the reactor has a reflux column and a reflux condenser. 